Dual range servomotor responsive to fluid pressure variations



March 2, 1954 A. R. WORSTER ET AL 2,670,716

DUAL RANGE SERVOMOTOR RESPONSIVE TO FLUID P ONS Filed RESSURE VARIATI Oct. 20, 1951 VENTORS ARTHUR R. WORSTER CLYDE E. HOLVENSTOT THEIR ATTORNEY Patented Mar. 2, 1954 DUAL RANGE SERVOMOTOR RESPONSIYE TO FLUID PRESSURE VARIATIONS Arthur R. Worster, Painted Post, and Clyde E.

Holvenstot, Corning, N. Y., assignors to Ingersoll-Rand Company, New York, N. Y., a corporation of New Jersey Application October 20, 1951, Serial No. 252,297

5 Claims.

This invention relates to servo-motors, and

more particularly to a servo-motor adapted to act in response to slight variations in pressure of a pilot fluid for controlling the movement of a power member capable of exerting a force of far greater magnitude than such pressure variations;

It is, accordingly, one object of this invention to construct a servo-motor which is responsive to slight variations in fluid pressure for controlling a force of greater magnitude than such variations in the pressure fluid.

A further object of this invention is to provide a servo-motor in which movement of the power member is directly proportional to the variations in pressure of the pilot fluid, and in which this proportion is of one value below some predetermined pressure of the pilot fluid, and a different value whenever the pressure of the pilot fluid is above said predetermined value.

Further objects of this invention will become obvious from the following specification accompanied by a drawing, in which:

Figure 1 is a longitudinal sectional view of a preferred form of the invention,

Figures 2, 3, 4 and 5 are transverse views taken through Fig. 1 looking in the direction of the arrows.

Referring to Figure l, a servo-motor constructed in accordance with the practice of the invention comprises, in general, a casin defining a power chamber II and a pilot chamber IZ in which reciprocates, respectively, a power member I3 and a pilot member I4. The pilot member, or piston, I4 is normally constantly urged in one direction (downward as viewed in Figure 1) throughout the entire cycle of the piston I4 by pilot pressure fluid conducted to one end of the pilot chamber I2 through a passage I5; Means-such as the spring l1are provided to exert a force on the piston I4 in the same direction as that exerted by pressure fluid duringpart only of the cycle of the piston. These forces are opposed by a spring. I6 which acts, as does the spring I7, against the piston rod I8 of the piston I4. With this arrangement, whenever the pressure Within the chamber I2 increases, the piston It moves downward compressing the spring I6 and relaxing spring I! until the force exerted by the sprin I6 is equal to thecombined forces exerted by the spring ['1' and pressure fluid acting on the piston M. A decrease in pilot pressure results in upward movement of the piston I4 under the influence 2 of the spring t6; the spring H is, of course, compressed by such movement.

Movement of the piston l4 serves to control the supply of power to actuate the power piston I3. More particularly, pressure fluid is conduct-- ed to one end of the power chamber II through a passage f9 to actuate the piston in one direc tion. A spring 35, or other compressible means, urges the piston I3 in the opposite direction. The pilot piston I4 controls the pressure of fluid acting against the piston I3 by varying the rate of flow of such fluid out of the power chamber through an outlet 20.

Referring in greater detail to the construction of the servo-motor, the piston I4 is mounted intermediate the ends of the piston rod Ill. The upper end portion H of the rod I8 extends slidably through an end cover 22 and is supported at its extreme end portion by a guide member 23 mounted on bolts 24 threaded in the cover 22. The spring I6 is biased between the end cover 22 and a spring retainer 25 threaded on the piston rod I8; and the spring l! is biased between the spring retainer 25 and a sleeve 21 threaded in the guide member 23 to oppose the force exerted by the spring IT.

The opposite, or lower, end portion 26 of the rod I8 extends slidably through a central perforation 28 in a cylindrical member 29 forming a partition between the power and pilot chambers II and [2, respectively. This end portion 26 of the piston rod I8 serves as a valve means and in furtherance to this end is arranged coaxially with the outlet passage 20 formed in the piston I3 and piston rod 31 and communicating the power chamber portions 30 and 3| above and below, respectively, the piston I3. Threaded in the mouth of the outlet 20 is a bushing 32 forming a seat for the lower end of the rod 18 such that movement of the rod l8 toward its seat 33 serves to reduce the effective flow area through the passage 20, and movement of the rod away from the seat 33 increases such flow area. The portion 3| of the chamber II is communicated with the atmosphere through a port 34 so that movement of the rod I8, relative to the piston I3, controls the escape of power fluid from the power chamber; and inasmuch as the flow of pressure fluid into the power chamber is limited by an orifice 4B in a plug 4| threaded in the passage l9, such movement controls the pressure value of fluid in the portion 30 of the chamber II.

Downward movement of the power piston under the force of pressure fluid in the power chamber is opposed by the spring 35 biased between the lower surface of the piston I3 and an end cover 36 mounted on the end of the casing l and arranged to support and guide the piston rod 37. The lower end of the rod 3'! extends through the cover 36 and is adapted to be connected to the device to be operated (not shown).

A passage 38 formed in the partition 29 communicates the lower end of the chamber I2 with the atmosphere to permit the escape of pressure fluid leaking into this portion of the chamber I2 from the power chamber II along the piston rod I3 or from the upper portion of the pilot chamber past the pilot piston I4. The passage I9 is also formed in the partition 28. The passage I5 for conveying pilot pressure fluid to the chamber I2 from a variable pressure source (not shown) is formed in the end cover 22. A leak passage 39 also formed in the end cover 22 conveys any leakage of pressure fluid from the chamber I2 along the portion of the shaft extending through the cover 22 to the atmosphere.

In the operation of the servo-motor, and referring first only to the forces acting on the pilot piston Id, assume that the fluid pressure force acting against the piston I4 exerts a downwardly directed force of ten pounds. If the piston I4 is at or near the upper end portion of the chamber I2such a position as is shown in Fig. 1-the spring I! will also exert a downwardly directed force, say for example five pounds, on the rod I8. Accordingly the downwardly directed net force is fifteen pounds. This force will be opposed by a fifteen pound force exerted in the upwardly direction on the piston I l by the spring I6. In the event that the pressure of the pilot fluid varies so that the force exerted on the piston by such fluid increases to say twenty pounds, the piston I4 will move downward compressing the spring I6 and permitting extension of the spring I1. For the sake of example, it will be assumed that the piston I4 moved one inch and that the force exerted by the spring I! was reduced to two pounds. At this point the downwardly directed net force is twenty-two pounds. Accordingly, the spring l6 exerts, due to its compressing, an upwardly directed force of twentytwo pounds.

Given these conditions, the position of the piston I l and the corresponding spring forces can be determined for any pilot pressure. Moreover, it can be shown that once the spring IT is fully extended, a smaller change in pressure is required to move the piston I4 a given distance than when the spring I1 is compressed. This is perhaps best illustrated by carrying out further and analyzing the example given. The forces exerted by the springs l3 and I1 and the piston M for various positions of the valve 23 are as follows:

Total Force of Force of Force of Distance Position Piston 14 Spring 17 Spring 16 Piston 14 Moves tion A. It is to be noted that for this movement (A to C) of 1 inches, during which the spring I! was under compression, the force exerted by the piston I4 was increased 16% pounds. However, with the spring I! completely extended (position C) it requiries an increase in piston force of only 11 pounds (l% '7) to move the piston 1 inches (position D). That is, it is necessary only to overcome the force exerted by the spring I 6. It follows, therefore, that a smaller force (or pressure change in chamber I2) is required to move the piston I4 a given distance after the spring I! is completely extended than is required to move the piston the same distance when the spring I! is under compression. In other words, the spring l6 and pressure fluid acting against the piston I4 serve as means for exerting opposing forces against the piston throughout its entire stroke or cycle, whereas the spring ll exerts a force in the same direction as the pilot fluid during a portion only of the piston stroke.

Referring now to the effect of the movement of the rod I8 on the operation of the power piston I3, and assuming, initially, that the spring 35 exerts an upwardly directed force of thirty pounds on the rod 31 and that the pressure of the power fluid in the power chamber II is sufficient to exert a downwardly directed force of thirty pounds on the rod 31, movement of the rod I8 downwardly towards the seat 33 decreases the flow area through the passage 20 thereby increasing the pressure within the chamber portion 30. This increase in pressure causes the piston I3 to move downwardly compressing the spring 35. So long as the rod I8 moves downwardly restricting the flow of power fluid through the passage 29, the piston I3 will be forced downwardly ahead of the rod I8. When the movement of the rod I8 is halted, due to a balance of the forces previously mentioned, the piston I3 will assume a position, relative to the end of the rod I8, such that the flow area through the passage 20 maintains a pressure within the power chamber 3| sufficient to exert a force on the piston I3 equal to the opposing force exerted by the spring 35.

Movement of the pilot piston upwardly, such that the end of the rod [8 moves away from its seat 33, increases the flow area through the passage 20 so that the pressure of the power fluid in the power chamber II decreases, the spring 35 will then move the piston I3 upwardly towards the end of the rod I3 until the flow area through the passage 23 is again decreased to increase the pressure of power fluid and es-- tablish an equilibrium of forces on the piston I3.

In the example, it was assumed that the pressure within the power chamber II was suflicient to exert a downwardly directed force of thirty pounds on the rod 31. In practice, this pressure may vary over a considerable range and control a force of three hundred pounds as easily as a force of thirty pounds. That is, the pressure within the power chamber does not in any manner effect the operation of the pilot piston, beyond the slight fluid pressure force exerted on the lower end of the piston rod I8.

In describing the operation of the servo-motor, it was assumed that the pressure at the source communicated with the pilot chamber l2 varied over a range of positive pressures only. The operation of the servo-motor is not, however, dependent on'such a pressure condition, but will operate equally effectively where the pilot pressure varies from a negative value, or vacuum, to a positive value.

Under such a condition, whenever the pilot pressure is less than atmospheric, the force of the spring l6 and the force of atmospheric pressure acting on the lower surface of the pilot piston l4 urges the piston 14 in one direction and these forces are opposed by the spring ll. When the pilot pressure reaches a positive value, 1. e., a pressure value above atmospheric pressure, the pilot pressure then exerts a downwardly directed force on the pilot piston M in the manner previously discussed. Thus whether the pilot pressure at the source is a positive or negative value, it serves to exert a force on the pilot piston through the entire piston cycle.

It is to be noted that inasmuch as the spring retainer is adjustably mounted on the piston rod l8-the retainer 25 is threaded on the rod l8-the pressure at which the servo-mechanism comes into operation may be readily adjusted by merely rotating the spring retainer 25 on the rod l8 to vary the force exerted by the spring it. Moreover, by adjusting the sleeve 21, relative to the spring retainer 25, the spring I! can be made to reach complete extension at any desired point in the piston cycle. In other words, the latter adjustment permits the change in ratio, hereinbefore referred to, between variations in pilot pressure and movement of the power piston to be made to occur at any pilot pressure value desired.

While we have shown and described a. specific form of our invention, it is to be understood that various changes and modifications may be made without departing from the spirit of the invenfor constantly urging the pilot member in one direction throughout the entire cycle of the pilot member, means for urging the pilot member in the opposite direction, and means urging the pilot member in the same direction as the second said means during part only of the cycle of the pilot member.

2. A servo-motor comprising a casing, a power member in the casing, means for continuously supplying power to the casing to exert a force on the power member in one direction, compressible means for opposing said force, a pilot member in the casing, valve means connected to said pilot member and arranged to vary said force, means for constantly urging the pilot member in one direction throughout the entire stroke of the pilot member, means for opposing the force exerted on the pilot member by the last said means, and means urging the pilot member in the same direction as the third said means during part only of the cycle of the pilot member.

3. A servo-motor comprising a casing having a power chamber and a pilot chamber therein, a power piston reciprocable in the power chamber, a passage for continuously supplying pressure fluid to the power chamber to actuate the power piston, a spring arranged to oppose movement or the power piston by the pressure fluid,

a pilot piston reciprocable in the pilot chamber, a valve connected to the pilot piston and arranged to control the pressure of fluid in the power chamber, means for constantly urging the pilot member in one direction throughout the entire stroke of the pilot member, a spring arranged to urge the pilot piston in the opposite direction, and means urging the pilot member in the same direction as the last said means during part only of the cycle of the pilot piston.

4. A servo-motor comprising a casing having a pilot chamber and a power chamber therein, a power piston reciprocable in the power chamber and adapted to be connected to the device to be actuated, a spring constantly urging the power piston in one direction, a passage for supplying pressure fluid to the power chamber for actuating the power piston in the opposite direction, an outlet for the power chamber, a pilot piston in the pilot chamber, valve means connected to the pilot piston and arranged to control the rate of flow through said outlet for varying the pressure of fluid in the power chamber, a conduit for conducting pressure fluid to the pilot chamber for actuating the pilot piston in one direction, a spring connected to urge the pilot piston in the same direction during part only of the stroke of said piston, and a spring constantly urging the pilot piston in the opposite direction.

5. A servo-motor comprising a casing having a pilot chamber and a power chamber therein, a power piston reciprocable in the power chamber and adapted to be connected to the device to be actuated, a spring constantly urging the power piston in one direction, a passage for supplying pressure fluid to the power chamber for actuating the power piston in the opposite direction, means for restricting the flow of pressure fluid through said passage, an outlet formed in the power piston to permit the escape of pressure fiuid from the power chamber, a pilot piston in the pilot chamber, a valve connected to the pilot piston and arranged such that movement of the pilot piston controls the rate of flow through said outlet to vary the pressure of fluid in the power chamber, a conduit for communicating the pilot chamber with a pressure source, a spring for urging the pilot piston in one direction during at least part of the piston stroke, and a spring constantly urging the pilot piston in the opposite direction throughout the entire stroke of the pilot piston.

ARTHUR R. WORSTER. CLYDE E. HOLVENSTOT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,141,975 Osborne June 8, 1915 1,585,529 Boving May 18, 1926 2,020,847 Mitereff Nov. 12, 1935 2,153,381 Maas Apr. 4, 1939 2,353,610 Chisholm, Jr. July 11, 1944 2,411,747 Nelson Nov. 26, 1946 2,418,129 Larson Apr. 1, 1947 2,486,047 Marinelli Oct. 25, 1949 FOREIGN PATENTS Number Country Date 295,290 Great Britain Oct. 31, 1929 

